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United States Patent |
5,679,171
|
Saga
,   et al.
|
October 21, 1997
|
Method of cleaning substrate
Abstract
A method of cleaning the surface of a substrate, wherein the first step of
the process of cleaning the substrate including a step of cleaning the
surface of the substrate by an acidic solution, oxidizing solution, or
alkaline solution, comprises a step of removing the natural oxide film
formed on the surface of the substrate.
Inventors:
|
Saga; Koichiro (Kanagawa, JP);
Koyata; Sakuo (Kanagawa, JP);
Hattori; Takeshi (Kanagawa, JP)
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Assignee:
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Sony Corporation (JP)
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Appl. No.:
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611673 |
Filed:
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March 6, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
134/3; 134/2; 134/26; 134/28; 134/29; 257/E21.228 |
Intern'l Class: |
C03C 023/00; B08B 003/00 |
Field of Search: |
134/2,1.3,26,3,28,29,27
|
References Cited
U.S. Patent Documents
4795497 | Jan., 1989 | McConnell | 134/18.
|
4921572 | May., 1990 | Roche | 139/2.
|
5454901 | Oct., 1995 | Tsuji | 134/3.
|
5487398 | Jan., 1996 | Ohmi et al. | 134/95.
|
5571375 | Nov., 1996 | Izumi et al. | 134/3.
|
Primary Examiner: Warden; Robert J.
Assistant Examiner: Markoff; Alexander
Attorney, Agent or Firm: Kananen; Ronald P.
Claims
What is claim is:
1. A method of cleaning a surface of a substrate, wherein a first step of a
process of cleaning the substrate comprises a step of removing a natural
oxide film formed on the surface of the substrate, including, after the
step of removing the natural oxide film, at least
a second step of cleaning by an alkaline solution,
a third step of cleaning by an acidic solution, and
a fourth step of wet etching to remove an SiO film formed as a result of
said third cleaning step, and
wherein the alkaline solution comprises ammonia, hydrogen peroxide, and
pure water.
2. A method of cleaning a substrate as set forth in claim 1, wherein the
step of removing the natural oxide film comprises immersing the substrate
in a solution containing hydrofluoric acid.
3. A method of cleaning a substrate as set forth in claim 2, wherein the
hydrofluoric acid solution has a concentration of 0.5 percent by weight
and wherein the time of immersion is approximately 1 minute.
4. A method of cleaning a substrate as set forth in claim 2, wherein
solution containing hydrofluoric acid comprises a buffered hydrofluoric
acid solution containing hydrofluoric acid and ammonium fluoride.
5. A method of cleaning a substrate as set forth in claim 4, wherein the
buffered hydrofluoric acid comprises 0.1 percent by weight of hydrofluoric
acid and 60 percent by weight of ammonium fluoride.
6. A method of cleaning a substrate as set forth in claim 1, wherein the
step of removing the natural oxide film comprises exposing the substrate
in a vapor containing hydrofluoric acid.
7. A method of cleaning a substrate as set forth in claim 6, wherein the
vapor containing hydrofluoric acid comprises a vapor of buffered
hydrofluoric acid containing hydrofluoric acid and ammonium fluoride.
8. A method of cleaning a substrate as set forth in claim 7, wherein said
vapor of buffered hydrofluoric acid is formed by heating an aqueous
solution of 0.1 percent by weight of hydrofluoric acid and 60 percent by
weight of ammonium fluoride.
9. A method of cleaning a substrate as set forth in claim 8, wherein the
pressure of the vapor of buffered hydrofluoric acid is made ordinary
pressure.
10. A method of cleaning a substrate as set forth in claim 9, wherein the
silicon substrate is exposed in the vapor containing the buffered
hydrofluoric acid for 0.5 to approximately 1 minute.
11. A method of cleaning a substrate as set forth in claim 1, wherein said
wet etching step is performed using hydrofluoric acid or buffered
hydrofluoric acid.
12. A method of cleaning a substrate as set forth in claim 2, wherein said
wet etching step is performed using hydrofluoric acid or buffered
hydrofluoric acid.
13. A method of cleaning a substrate as set forth in claim 6, wherein said
wet etching step is performed using hydrofluoric acid or buffered
hydrofluoric acid.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of cleaning a substrate, more
particularly relates to a method of cleaning a substrate which enables the
organics or inorganics or other contaminants adhering to the surface of
the substrate to be effectively removed from the substrate surface.
2. Description of the Related Art
In the process of production of a semiconductor device, contamination can
occur having a detrimental effect on the operating characteristics of the
semiconductor device. One type of contamination is the minute amount of
organics adsorbed on the surface of the silicon substrate. This minute
amount of organics ends up adsorbed to the surface of the semiconductor
device even when lust allowing the silicon substrate to stand in a
dust-free atmosphere or plastic box.
In the cleaning methods of the related art, it is not possible to
effectively remove the minute amount of organics adsorbed to the surface
of the silicon substrate. Therefore, the minute amount of organics which
could not be removed remain on the surface of the silicon substrate and
suffers from the disadvantage on production.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of cleaning a
substrate enabling removal of even the minute amount of adsorbed organics
present on the substrate surface.
To achieve the above object, the present invention provides a method of
cleaning the surface of a substrate wherein the first step of the process
of cleaning the substrate, including a step of cleaning the surface of the
substrate by an acidic solution, oxidizing solution, or alkaline solution,
is a step of removing the natural oxide film formed on the surface of the
substrate.
Preferably, the natural oxide film is removed by immersing the substrate in
a solution containing hydrofluoric acid. The solution containing
hydrofluoric acid may be a hydrofluoric acid solution of a concentration
of 0.5 percent by weight and the time of immersion may be approximately i
minute. Alternatively, the solution containing hydrofluoric acid may be a
buffered hydrofluoric acid solution containing hydrofluoric acid and
ammonium fluoride. In this case, the buffered hydrofluoric acid solution
preferably is comprised of 0.1 percent by weight of hydrofluoric acid and
60 percent by weight of ammonium fluoride.
Preferably, the alkaline solution is comprised of ammonia, hydrogen
peroxide, and pure water.
Alternatively, the natural oxide film may be removed by exposing the
substrate in a vapor containing hydrofluoric acid. In this case,
preferably the vapor containing hydrofluoric acid is a vapor of a buffered
hydrofluoric acid containing hydrofluoric acid and ammonium fluoride.
Preferably, the vapor of the buffered fluoride acid is formed by heating
an aqueous solution of 0.1 percent by weight of hydrofluoric acid and 60
percent by weight of ammonium fluoride. In this case, the pressure of the
vapor of the buffered hydrofluoric acid is preferably made ordinary
pressure and the silicon substrate is preferably exposed in the vapor
containing the buffered hydrofluoric acid for 0.5 to approximately 1
minute.
Preferably, after the step of removing the natural oxide film, there are
further provided a step of cleaning by an alkaline solution and a step of
cleaning by an acidic solution.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will become
apparent from the following description of the preferred embodiments made
with reference to the attached drawings, in which
FIG. 1 is a flow chart of the method of cleaning a silicon substrate
according to the related art,
FIG. 2 is a flow chart of the method of cleaning a silicon substrate
according to an embodiment of the present invention,
FIGS. 3A to 3D are schematic cross-sectional views of the state of the
surface of a substrate at the steps of the method of cleaning a silicon
substrate according to an embodiment of the present invention, and
FIG. 4 is a graph comparing the cleaning effect of a substrate by the
method of cleaning a silicon substrate according to embodiments of the
present invention and the cleaning effect of the methods of cleaning
according to the related art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing the present invention, a brief explanation will be made
of one of the related arts by way of reference.
The surface of silicon substrates has been cleaned in the related art by
the process shown in the flow chart of FIG. 1.
As shown in FIG. 1, first, at step 1, the surface is cleaned using a strong
acid or a strong oxidizing agent (sulfuric acid+hydrogen peroxide, fuming
nitric acid, ultra-pure water with ozone, etc.) for the purpose of
removing the organics from the surface of the silicon substrate. Next, at
step 2, pure water is used to rinse the surface to wash off the cleaning
solution of the previous step, then at step 3 the surface is cleaned using
an alkaline cleaning solution (ammonia+hydrogen peroxide +pure water).
Next, at step 4, the surface is rinsed and cleaned with pure water to wash
off the cleaning solution of the previous step, then at step 5 the surface
is cleaned using an acidic cleaning solution (hydrochloric acid+hydrogen
peroxide+pure water etc.) for the purpose of removing the heavy metals.
Next, at step 6, the surface is rinsed and cleaned by pure water to wash
off the cleaning solution of the previous step, then at step 7, the
surface is wet etched using hydrofluoric acid or buffered hydrofluoric
acid for the purpose of removing the natural oxide film formed on the
silicon substrate surface by the cleaning solution. Next, at step 8, the
surface of the silicon surface is stripped of water and dried.
In the cleaning method of the above related art, however, the present
inventors found by experiments that it was not possible to effectively
remove the minute amount of organics adsorbed to the surface of the
silicon substrate. That is, while it was possible to reduce the large
amount of adsorbed organics to a minute amount by the cleaning by the
strong acid or strong oxidizing agent applied first in the above cleaning
method of the related art, it was not possible to reduce the minute amount
of the adsorbed organics to substantially close to zero.
Note that in the process of the related art, the treatment by the
hydrofluoric acid or buffered hydrofluoric acid was performed as a later
step of the cleaning by the acidic solution, but this treatment was for
removing the oxide film formed on the surface of the silicon by the
cleaning by the oxidizing solution. Therefore, the minute amount of
organics which could not be removed by the strong acid or strong oxidizing
agent remains on the surface of the silicon substrate even after the
stripping and drying treatment of step 8 and becomes a problem in
production.
The present invention provides a method of cleaning the surface of a
substrate wherein the first step of the process of cleaning the substrate
including a step of cleaning the surface of the substrate by an acidic
solution, oxidizing solution, or alkaline solution, comprises a step of
removing the natural oxide film formed on the surface of the substrate.
The natural oxide film is removed by immersing the substrate in a solution
containing hydrofluoric acid or exposing the substrate in vapor containing
hydrofluoric acid. As the solution containing the hydrofluoric acid, use
may be made of an aqueous solution of hydrofluoric acid, a buffered
hydrofluoric acid solution containing hydrofluoric acid and ammonium
fluoride, etc. As the vapor containing the hydrofluoric acid, use may be
made of a vapor of an aqueous solution of hydrofluoric acid, a vapor of a
buffered hydrofluoric acid containing hydrofluoric acid and ammonium
fluoride, etc.
When using a hydrofluoric acid, the concentration of the hydrofluoric acid
is preferably at least 0.1 percent by weight and the treatment time is for
example 30 seconds to 3 minutes. When using a buffered hydrofluoric acid,
the concentration of the hydrofluoric acid is 0.05 to 10 percent by
weight. The ratio of weight of the ammonium fluoride (NH.sub.4 F) to the
hydrofluoric acid (HF) is about 1:200 to 600. The treatment time is for
example 30 seconds to 3 minutes.
After the step of removing the natural oxide film, at least a step of
cleaning by an alkaline solution and a step of cleaning by an acidic
solution are performed.
The substrate to be cleaned by the method of cleaning of the present
invention is not particularly limited, but for example, may be a silicon
substrate, gallium arsenide substrate, or other semiconductor substrate or
other substrate.
In the method of cleaning a substrate according to the present invention,
the first step of cleaning is to remove the natural oxide film formed on
the surface of the substrate by hydrofluoric acid, buffered hydrofluoric
acid, etc., so it is possible to effectively remove the minute amount of
organics and inorganics adsorbed on the natural oxide film.
Next, it is possible to remove the particulates on the substrate surface by
cleaning by an alkaline solution (ammonia+hydrogen peroxide+pure water).
Further, it is possible to remove the heavy metals on the surface of the
substrate by cleaning by an acidic solution (hydrochloric acid+hydrogen
peroxide+pure water etc.)
When cleaning the substrate by the acidic solution, an oxide film is
sometimes formed on the surface of the substrate, but this film can be
removed by a final step of wet etching by hydrofluoric acid, buffered
hydrofluoric acid, etc.
As a result, it is possible to substantially completely remove the organics
adsorbed on the surface of the substrate before cleaning and possible to
easily obtain a substrate with excellent Surface conditions. That is, the
method of cleaning of the present invention enables one to obtain an
extremely clean semiconductor wafer or other substrate, eliminates the
contamination by the minute amount of organics in the later film-forming,
etching, exposure, and other processes, and thereby enables production of
extremely high quality semiconductor devices with a good yield.
In particular, silicon wafers and other substrates are sometimes placed in
plastic boxes for storage or transportation etc. The plasticizers,
cross-linking agents, antioxidants, etc. contained in the plastic
sometimes are absorbed on the oxide film on the surface of the substrates.
In the method of cleaning of the related art, it had not been possible to
effectively remove these minute amounts of organics from the substrate
surface.
According to the method of cleaning of the present invention, it is
possible to substantially completely remove the organics adsorbed on the
surface of the substrates before cleaning. The minute amounts of organics
able to be removed by the method of the present invention are not
particularly limited, but include, for example, substances easily adhering
to natural oxide films such as diacetylbenzene and other cross-linking
agents, dibutyl phthalate (DBP), dioctyl phthalate, and other
plasticizers, di-tertiary-butyl-p-cresol (BHT), and other antioxidants.
The method of cleaning a substrate according to the present invention will
be explained in further detail below based on the embodiments shown in the
figures.
FIG. 2 is a flow chart of the method of cleaning a silicon substrate
according to an embodiment of the present invention, FIGS. 3A to 3D are
schematic cross-sectional views of the state of the surface of a substrate
at the steps of the method of cleaning a silicon substrate according to an
embodiment of the present invention, and FIG. 4 is a graph comparing the
cleaning effect of a substrate by the method of cleaning a silicon
substrate according to embodiments of the present invention and the
cleaning effect of the methods of cleaning according to the related art.
FIRST EMBODIMENT
Next, a detailed explanation will be made of a first embodiment of the
present invention.
Various contaminants are present on the surface of a silicon substrate
before it is subjected to a film-forming, photolithographic, or other
process. The state of adsorption of the contaminants is, for example, as
shown in FIG. 3A. That is, a natural oxide film 13 containing metal
impurities 14 is present on the surface of the silicon substrate 11.
Further, organics 12 are adsorbed to that surface.
If the organics 12 are not removed in the first step of the cleaning of the
surface of the silicon substrate, the surface will remain covered by the
organics 12 and therefore it will be difficult to remove the heavy metals
and other metal impurities 14 in the later steps.
Therefore, in this embodiment, as shown in FIG. 2, in the first step of the
cleaning, that is, step 20, the silicon substrate is immersed in an
aqueous solution of hydrofluoric acid. The aqueous solution of
hydrofluoric acid has a concentration of hydrofluoric acid (HF) of 0.5
percent by weight (0.25M/liter) in this embodiment. The substrate is
immersed for approximately 1 minute.
As a result of the treatment by hydrofluoric acid, as shown in FIG. 3B, the
adsorbed organics 12 can be removed together with the natural oxide film
13. The metal impurities 14 remain on the surface of the silicon
substrate.
An experiment was performed as follows to show that the impurities adsorbed
on the surface of the silicon substrate could be substantially completely
removed by the treatment by hydrofluoric acid.
The hydrofluoric acid-treated silicon substrate was heated to 400.degree.
C., the desorbed gas was condensed, and the content of the adsorbed
organics was analyzed by gas chromatography and mass spectrophotometry
(GC/MS). The results are shown in FIG. 4. In FIG. 4, the vertical axis
shows the content of the diacetylbenzene and dibutyl phthalate (DBP). The
horizontal axis shows the methods of cleaning of the related art and
present invention. As shown in FIG. 4, it was confirmed that the present
invention enabled the approximately 210 ng content of the diacetylbenzene
and the approximately 20 ng content of the dibutyl phthalate to be
substantially completely removed.
As opposed to this, the related arts of O.sub.2 Asher, cleaning by an
aqueous solution of ammonia+hydrogen peroxide (APM), cleaning by an
aqueous solution of sulfuric acid+hydrogen peroxide (SPM), and cleaning by
fuming nitric acid (HNO.sub.3) were not able to completely remove the
organics.
In the present embodiment, after the treatment by the hydrofluoric acid,
the substrate is rinsed by pure water tow ash off the cleaning solution of
the previous step at step 21 of FIG. 2, then is cleaned using an alkaline
(ammonia+hydrogen peroxide+pure water, etc.) cleaning solution for the
purpose of removing particulates at step 22.
Next, at step 23, the substrate is rinsed and cleaned by pure water to wash
off the cleaning solution of the previous step, then at step 24 it is
cleaned using an acidic (hydrochloric acid+hydrogen peroxide+pure water
etc.) cleaning solution. The cleaning by this acidic solution removes the
metal impurities 14 from the surface of the silicon substrate 11 shown in
FIG. 3B. However, the cleaning by the acidic solution, as shown in FIG.
3C, results in the formation of a clean natural silicon oxide film 15 on
the surface of the silicon substrate 11.
Next, at step 25 shown in FIG. 2, the surface is rinsed and cleaned by pure
water to wash off the cleaning solution of the previous step, then at step
26 hydrofluoric acid or buffered hydrofluoric acid is used to wet etch the
substrate surface. This wet etching, as shown in FIG. 3D, enables the
removal of the natural oxide film 15 formed on the surface of the silicon
substrate by the cleaning solution. Next, at step 27 shown in FIG. 2, the
surface of the silicon substrate is stripped of water and dried.
In the method of cleaning of a substrate according to this embodiment,
since the first step of cleaning is to remove the natural oxide film
formed on the surface of the substrate by hydrofluoric acid, it is
possible to efficiently remove the minute amount of adsorbed organics
adhering to the natural oxide film.
SECOND EMBODIMENT
Next, an explanation will be made of a second embodiment of the present
invention.
The same procedure is followed as in the first embodiment to clean the
silicon substrate except that use is made of buffered hydrofluoric acid
instead of hydrofluoric acid at step 20 shown in FIG. 2. The buffered
hydrofluoric acid used is an aqueous solution of 0.1 percent by weight of
hydrofluoric acid and 60 percent by weight of ammonium fluoride. The
substrate is immersed for approximately 1 minute.
In the same way as the case of the first embodiment, the content of the
organics remaining on the surface of the silicon substrate after treatment
by the buffered hydrofluoric acid was examined. As shown in FIG. 4, it was
almost 0.
In the method of cleaning of a substrate according to this embodiment,
since the first step of cleaning is to remove the natural oxide film
formed on the surface of the substrate by buffered hydrofluoric acid, it
is possible to efficiently remove the minute amount of adsorbed organics
adhering to the natural oxide film.
THIRD EMBODIMENT
Next, an explanation will be made of a third embodiment of the present
invention.
The same procedure is followed as in the first embodiment to clean the
silicon substrate except that use is made of a vapor of hydrofluoric acid
instead of a solution of hydrofluoric acid at step 20 shown in FIG. 2. The
vapor of hydrofluoric acid used is obtained by heating an aqueous solution
of 0.5 percent by weight of hydrofluoric acid (0.25M/liter). The
temperature of the vapor is ordinary temperature and the pressure is
ordinary pressure. The substrate is exposed to the vapor for 0.5 to
approximately 1 minute.
In the same way as the case of the first embodiment, the content of the
organics remaining on the surface of the silicon substrate after treatment
by the vapor of hydrofluoric acid was examined. As shown in FIG. 4, it was
almost 0.
In the method of cleaning of a substrate according to this embodiment,
since the first step of cleaning is to remove the natural oxide film
formed on the surface of the substrate by vapor of hydrofluoric acid, it
is possible to efficiently remove the minute amount of adsorbed organics
adhering to the natural oxide film.
FOURTH EMBODIMENT
Next, an explanation will be made of a fourth embodiment of the present
invention.
The same procedure is followed as in the first embodiment to clean the
silicon substrate except that use is made of a vapor of buffered
hydrofluoric acid instead of the solution of hydrofluoric acid at step 20
shown in FIG. 2. The vapor of buffered hydrofluoric acid used is obtained
by heating an aqueous solution of 0.1 percent by weight of hydrofluoric
acid and 60 percent by weight of ammonium fluoride. The temperature of the
vapor is ordinary temperature and the pressure is ordinary pressure. The
substrate is exposed to the vapor for 0.5 to approximately 1 minute.
In the same way as the case of the first embodiment, the content of the
organics remaining on the surface of the silicon substrate after treatment
by the vapor of buffered hydrofluoric acid was examined. As shown in FIG.
4, it was almost 0.
In the method of cleaning of a substrate according to this embodiment,
since the first step of cleaning is to remove the natural oxide film
formed on the surface of the substrate by vapor of buffered hydrofluoric
acid, it is possible to efficiently remove the minute amount of adsorbed
organics adhering to the natural oxide film.
For example, the treatment performed at step 20 shown in FIG. 2 is not
limited to treatment by immersing the substrate in a solution containing
hydrofluoric acid or treatment by exposing the substrate in vapor
containing hydrofluoric acid. Another treatment may be used so long as it
enables the natural oxide film formed on the surface of the silicon
substrate to be removed together with the organics on the surface.
As explained above, according to the present invention, it is possible to
substantially completely remove the organics adsorbed on the surface of
the substrate before cleaning and therefore possible to easily obtain a
substrate with an excellent surface state. That is, the method of cleaning
of the present invention enables one to obtain an extremely clean
semiconductor wafer or other substrate, eliminates the contamination by
the minute amount of organics in the subsequent film-forming, etching,
exposure, or other processes, and thereby enables production of extremely
high quality semiconductor devices with a good yield.
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